The "zone rouge" (in French, Red Zone) is a noncontiguous area that occupies the northeastern part of France and corresponds to the places that hosted some of the bloodiest battles of the Great War. Originally extending as much as 1,200 km2, over the years, thanks to reclamation efforts, it has been downsized to about 100 km2. Despite this, the subsoil still conceals a disproportionate amount of unexploded ordnance, conventional and otherwise, to the point that it remains virtually inaccessible to humans.

Sources:
https://pubmed.ncbi.nlm.nih.gov/17555...
https://www.messynessychic.com/2015/0...
https://www.lemonde.fr/alaune/artic...
https://www.riskope.com/2014/02/13/10...